A number of types of electromagnetic (EM) digitizers or digitizer tablets are known in the art. In general, a digitizer tablet provides an indication of a position of a puck, pen, or stylus with respect to a surface of a planar sensor grid. One type of digitizer tablet operates by sensing a magnetic field and includes two sets of array conductors, with one set being orthogonally oriented to the other set. In this type of system the pen is driven with an AC signal to cause an oscillating magnetic field which, in turn, induces signals in the array conductors. The signals in the array conductors are sensed and compared so as to determine the position of the pen in two dimensions, relative to the surface of the tablet.
Many digitizers utilize a single measurement channel to measure several loops or lines, from which measurements of the position of the pen may be interpolated. The interpolation algorithms are generally optimal for a specific set of loop measurements, relative to the position of the pen.
However, conventional digitizers generally either scan more loops than actually required, discarding the unneeded measurements, or scan loops expected to give the best results, and accept degraded performance when the pen is not in the expected position.
In U.S. Pat. No. 4,423,286, entitled "Apparatus and Method for Determining the Position of a Driven Coil Within a Grid of Spaced Conductors", Bergerson describes a scanning technique wherein a center conductor in the X direction is first scanned. By determining whether the polarity of a signal induced by a transmitting coil is positive or negative, it is determined in what half-section of the grid the transmitting coil is located. Next a center conductor in the determined half-section is scanned, then a center conductor in a determined quarter-section, etc., until the transmitting coil is located between two adjacent conductors. The exact coil position is determined by forming a ratio of the signal in the two adjacent conductors and comparing the ratio with values stored in a look-up table. A like set of measurements is then obtained for the conductors of the Y direction. As a result, a significant number of conductors must be scanned in each axis before the two adjacent conductors are located and before the exact coil determination technique can be employed. Furthermore, this approach is only applicable to scanning single conductors, wherein the induced signal, on one side of the pen, is guaranteed to be negative, and on the other side is guaranteed to be positive.
It is thus an object of the invention to provide methods for scanning sensing elements of a digitizing tablet so as measure a minimum number of sensing elements that are required to obtain an accurate interpolation or computation of a position of a field generating device.
It is another object of the invention to provide a method for scanning the inductors of an EM tablet so as measure only those inductors that are closest to a position of a transmitting coil and which are required to obtain an accurate interpolation of the position of the coil relative to the tablet.